Abstract
We present the first characterization of the spectral properties of superradiant light emitted from the ultra-narrow, 1 mHz linewidth optical clock transition in an ensemble of cold $^{87}$Sr atoms. Such a light source has been proposed as a next-generation active atomic frequency reference, with the potential to enable high-precision optical frequency references to be used outside laboratory environments. By comparing the frequency of our superradiant source to that of a state-of-the-art cavity-stabilized laser and optical lattice clock, we observe a fractional Allan deviation of $6.7(1)\times 10^{-16}$ at 1 second of averaging, establish absolute accuracy at the 2 Hz ($4\times 10^{-15}$ fractional frequency) level, and demonstrate insensitivity to key environmental perturbations.
Highlights
The development of atomic clocks has led to a wealth of applications, from technology to fundamental physics
Precision frequency metrology has been proposed as a means of studying quantum many-body physics [14,15], searching for exotic physics [16,17,18], and exploring fundamental quantum limits imposed by gravity [19,20,21]
We have demonstrated the first optical high-precision active atomic frequency reference to date
Summary
The development of atomic clocks has led to a wealth of applications, from technology to fundamental physics. Optical atomic clocks are currently the most precise and accurate absolute frequency references [1,2,3,4,5,6,7,8,9], while their microwave-domain counterparts are used to define the second [10,11] and for other practical applications, including communication and navigation [12,13]. The oscillator provides short-term stability, while the atomic frequency reference provides long-term stability and absolute accuracy. Hydrogen masers are currently used as active frequency references in the microwave domain to provide complementary short-term stability to microwave atomic clocks [22,27]. We demonstrate the first active optical frequency reference to realize this advantage, as proposed in Refs. [28,29], achieving a measured fractional frequency stability at short times surpassing that of hydrogen masers by roughly two orders of magnitude
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